TRILOBITE TAPHONOMY OF THE UPPERMOST DYERAN (LOWER CAMBRIAN) RUIN WASH LAGERSTÄTTE, CHIEF RANGE, NEVADA

The uppermost 43 cm of Dyeran strata at Ruin Wash contain exceptionally preserved olenelloid trilobites. Detailed sedimentology and ichnology shows that the strata were deposited as a series of event beds which progressively waned then waxed in intensity up section; benthic redox conditions shifted concurrently. Biostratinomic data for 550 trilobite specimens encountered in the measured section reveal that the fauna was autochthonous, although winnowed, bioclast-rich surfaces at the base and top of the section indicate intervals of prolonged pre-burial exposure time and in situ reworking. Trilobite biostratinomy was strongly influenced by the subtle environmental shifts: the degree of disarticulation and sclerite fracturing correlate positively with event bed energy and substrate oxygenation.

Patterns of sclerite disarticulation, inversion, rotation, and fracturing were documented for almost 1300 additional specimens from the site. Two-thirds of specimens recovered were isolated cephala. Of the 389 specimens preserving cephalic plus thoracic information, only 32 (8%) represent unequivocal carcasses and 11 (3%) represent unequivocal molts: the vast majority of articulated specimens are preserved in configurations which cannot be unambiguously interpreted in terms of death posture or ecdysis-related movement. The propensity for post-ecdysial or post-mortem modification of sclerite configurations by physical or biological processes cautions against interpreting particular configurations in terms of ecdysial movements unless they consistently preserve unambiguous and functionally logical molt signatures. Recognition of such consistent ensembles demands large sample size, but provides a rigorous basis for determining ranges of molting behavior.

Extensive cephalic fracturing is found only in intervals of relatively high reworking and longer sclerite duration in the taphonomically active zone. The frequency of sclerite fracturing in unambiguous carcasses does not significantly differ from that in unambiguous molts. Sclerite damage at Ruin Wash therefore resulted primarily from physical processes, and does not provide a reliable proxy for predation or scavenging. However, predation is indicated by rare instances of healed injuries.